python: extend and improve tests, fix minor glitches

python: extend and improve tests, fix minor glitches

* m4/python.m4 (AM_PATH_PYTHON): Ensure the "checking ..." messages
from configure are always complete, even in case of failure.  Tiny
cosmetic improvement in info/error messages.
* tests/python.test: Also check that automake complains if the
PYTHON primary is used but the `py-compile' script is not present.
Make grepping of generated Makefile.in laxer w.r.t. whitespace.
Add trailing `:' command.
* tests/python2.test: Remove repeated calls aclocal: they are
useless because configure.in is never modified.  Make grepping
of automake stderr more comprehensive.  Remove the pre-existing
`py-compile' file before trying to install it with `--add-missing'.
Add trailing `:' command.
* tests/python3.test: Add trailing `:' command.
* tests/python11.test: Likewise.
* tests/python4.test: Likewise.  Also, try to pass PYTHON config
variable to configure from the environment, rather than only from
the command line.
* tests/python5.test: Ensure that the "checking ..." messages from
configure are always complete.  Use proper m4 quoting.  Add a
trailing `:' command.
* tests/python6.test: Simplify test logic, by checking for files
created by configure rather then grepping its output.
* tests/python7.test: Likewise.
* tests/python8.test: Also check that `$PYTHON' is meaningfully
set in the ACTION-IF-TRUE argument of AM_PATH_PYTHON.
* tests/python9.test: Likewise.
* tests/python10.test: Add trailing `:' command.
(Makefile.am): Declare `disttest' target as `.PHONY', and add
an `ls -l' to its recipe, for debugging.
* tests/nobase-python.test: In testing "make uninstall" and
"make install" results, prefer idioms that make verbose logs
more helpful.  Remove a couple of lines of dead code.  Add a
trailing `:' command.
* tests/python5b.test: New test, checking that configure performs
the check on the python version even when the choice of the python
interpreter is forced by the user.
* tests/python-dist.test: New test, checking the distribution of
*_PYTHON files.
* tests/python-vars.test: New test, checking that AM_PATH_PYTHON
correctly set all the output variables advertised in the manual.
* tests/python-virtualenv.test: New test, checking that python
support offered by automake works well with "virtual python
environments" created by the `virtualenv' program.
* tests/instdir-prog.test: Also check `$(pyexecdir)'.  Existing
checks made slightly stricter.
* tests/instdir-prog.test: Also check `$(pyexecdir)'.  Existing
* tests/instdir-ltlib.test: Likewise.
* tests/Makefile.am (TESTS): Update.